Use of transition metal compounds in imageable coatings

ABSTRACT

A process for forming an image on a substrate, which comprises coati ng the substrate with an amine of molybdenum, tungsten or vanadium that changes colour on heating or irradiation as an aqueous dispersion or suspension or as a solution in an organic solvent. Also described is a coated substrate, wherein the coating is a substantially visible light-transparent layer comprising an amine compound of molybdenum, tungsten or vanadium, and a solution of said amine compound and a thermoplastic polymer or a photo-polymerisable monomer.

FIELD OF THE INVENTION

This invention relates to transition metal compounds and their use inimageable coatings.

BACKGROUND OF THE INVENTION

For many years, heat-sensitive imaging sheets have been used forcopying, thermal printing, thermal recording and thermal labelling. Morerecently, the development of scribing lasers has enabled the use ofthermally-sensitive imaging materials for the coding and marking of bothsheet materials and shaped objects that may or may not beself-supporting.

Two classes of colour-forming reactants have commonly been used forthermographic materials, i.e. leuco lactone or spiropyran compoundsnormally developed by phenolic compounds, e.g. as described in U.S. Pat.No. 3,846,153, and heavy metal salts of organic acids that can reactwith ligands to give coloured complexes, e.g. as described in U.S. Pat.No. 2,663,654. The use of both these types of compounds depends oneffecting a physical separation of the solid components, throughdispersing them in a polymer binder, coating them on a suitable support,and melting at least one of them to cause colour formation. When coatedand dried, dispersions of solid materials, by their nature, result inlayers of some opacity. This is normally acceptable on opaque substratessuch as paper, but limits applications on transparent substrates such asclear Mylar (polyester) film and transparent packaging films. Examplesof such applications are where a film transmission original is requiredor, in transparent film packaging applications, where film opacity wouldobscure sight of the packaging contents or container surface.

There is therefore a need for transparent, thermally-sensitive imaginglayers for coating on transparent or semi-transparent film supports andreflective supports such as can-metal. Further, there is a need fortransparent laser-sensitive imaging materials that may be coated orprinted on shaped or formed objects such as bottles and other containersfor labelling or coding applications. Naturally, for these applications,the coatings should adhere to the substrate firmly and be robust, i.e.have good resistance to the types of chemical and physical treatmentencountered in the end use environment. In general, organicsolvent-based compositions containing solvent-soluble binders give, ondrying, tougher, better adhering layers of greater transparency andwater-resistance than like water-based compositions.

The use of organic amine molybdates in thermal imaging layers isdescribed in U.S. Pat. No. 2,910,377 (see Example 10) and U.S. Pat. No.3,028,255 (where the exemplified amines are primary amines). This use isconfined to copy paper sheets, and the molybdate is dispersed byprolonged ball-milling in a resinous binder to give a suspension, usedfor coating. Such a suspension, when coated and dried on a transparentfilm support, would cause loss of transparency.

U.S. Pat. No. 4,217,409 (see Examples 10 and 12) describes the use ofisopropylammonium molybdate in an acidic aqueous solution of polyvinylalcohol as a coating that, when applied to a substrate, gives a laminarmaterial sensitive to electromagnetic radiation including IR, visibleand UV radiation. Polyvinyl alcohol solutions often have poor coatingproperties towards polyester film and the hazy dried films detachreadily. The dried and imaged coating would also be susceptible tophysical and chemical damage, most notably chemical damage from water.Isopropylamine is volatile and would cause odour should the material becontacted with aqueous alkali.

U.S. Pat. No. 4,406,839 describes the synthesis of organicsolvent-soluble amine molybdates useful as smoke retardants and madefrom a variety of amines. Examples employ high molecular weight aminessuch as tridodecylamine.

Amine molybdates, their synthesis and uses, are also described in U.S.Pat. No. 2,910,377, U.S. Pat. No. 3,028,255, U.S. Pat. No. 3,290,245,U.S. Pat. No. 4,053,455, U.S. Pat. No. 4,153,792, U.S. Pat. No.4,217,292, U.S. Pat. No. 4,217,409, U.S. Pat. No. 4,226,987, U.S. Pat.No. 4,266,051, U.S. Pat. No. 4,406,837, U.S. Pat. No. 4,406,838, U.S.Pat. No. 4,406,839, U.S. Pat. No. 4,406,840, U.S. Pat. No. 4,410,462,U.S. Pat. No. 4,410,463, U.S. Pat. No. 4,424,164, U.S. Pat. No.4,425,279, U.S. Pat. No. 6,217,797 and U.S. Pat. No. 6,355,277.

SUMMARY OF THE INVENTION

The present invention is based at least in part on the finding thatamine molybdates and analogous compounds, some of which may be new, haveproperties that render them suitable for imaging. In particular, theyare soluble in at least some organic solvents, are compatible withfilm-forming solvent-soluble organic binders, and give solutions that,when coated on an inert substrate such as clear polyester film anddried, form a continuous substantially visible light-transparent layeron the support. Such layers are thermally sensitive and find utility inthermographic materials for imaging by scanning laser or thermalprinter, to provide effective marking, without opacification in thenon-image areas.

According to one aspect of this invention, a process for forming animage on a substrate, comprises coating the substrate with a solution,in an organic solvent, of an amine compound of molybdenum, tungsten orvanadium, wherein the compound changes colour on heating or irradiation,and heating or irradiating the coating.

A further aspect of the invention is a coated substrate, wherein theamine is a secondary or tertiary alkylamine in which each alkyl grouphas up to 12 carbon atoms and the amine has up to 24 carbon atoms. Otheraspects are solutions of the amine compound and a photopolymerisablemonomer or a thermoplast.

The organic solvent solubility properties of the amine molybdates of theinvention permit the avoidance of the time-consuming, wasteful andcostly milling processes normally involved in the preparation of coatingmixtures for known thermally sensitive imaging materials. They alsoallow thermally sensitive layers of good transparency and gloss to bemade on transparent substrates such as Mylar and commercially availablepackaging films such as polypropylene.

DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred compounds for use in the invention are amine molybdates. Theterm “amine molybdate” (of which an example is ethylamine molybdate) isused herein to describe compounds whose structure may be ill-defined,and which are also sometimes called the corresponding ammoniummolybdates (e.g. ethylammonium molybdate), which implies that thecompounds are salts. The generic term “amine molybdate” refers tocomplexes or salts formed on reacting an amine to give an aminemolybdate or amine isopolymolybdate. For reference, see Cotton &Wilkinson; Advanced Inorganic Chemistry 2^(nd) Edition 1967 Chapter 30Section 30-C-2&3.

Amine molybdates will be described herein, for the purpose ofillustration. Such compounds for use in the invention may be formed fromamines and molybdate and polymolybdate (VI) acids and their salts andcan be can be thermally activated in a coating, to give an image. Othercompounds suitable for use in the invention, including those based ontungsten or vanadium, can be made in similar manner.

More specifically, such compounds are made, for example, using knownsaturated secondary or tertiary aliphatic dialkyl or trialkyl monoamineshaving boiling points (at 1 atmosphere pressure) equal to or above 150°C. and melting points below about 80° C., and with individual alkylgroups which are different or, preferably, the same, e.g. having from 3to 12, preferably 5 to 12, more preferably 5 to 10, and most preferably6 or 7 to 10 carbon atoms. Typically, the compound has a total of 7 to24 C atoms. Salts of the compounds may also be used, such as amineacetates or chlorides. Representative amines are dipentylamine,tripentylamine, di-n-hexylamine, tri-n-hexylamine,bis(2-ethylhexyl)amine, di-n-octylamine and tri-n-octylamine. It will beunderstood that one or more amine compounds may be used.

The amine molybdates are made by reacting the amine with a molybdenumcompound, e.g. in oxidation state VI, such as molybdenum trioxide,molybdic acid, ammonium dimolybdate, ammonium heptamolybdate, ammoniumoctamolybdate, sodium molybdate or commercial “molybdic acid” (whichcomprises primarily one or more ammonium molybdates). A representativeand preferred amine molybdate for use in the invention isbis(2-ethylhexyl)amine octamolybdate.

Amine molybdates suitable for use in the invention have one or more ofthe following properties:

-   -   (i) Individually soluble in at least one organic solvent    -   (ii) Transparent or near transparent film-forming properties on        specified commercial polymer substrates when applied by coating        or printing an organic solvent solution    -   (iii) Thermal sensitivity manifested as a colour change of good        visual discrimination when a layer comprising the amine        molybdate is exposed thermally imagewise by a scanning laser        and/or heat block imaged by a thermal printer    -   (iv) Compatibility with at least one solvent-soluble binder        polymer as indicated by the formation of a near transparent film        of a blend    -   (v) Preparation using an amine precursor of low volatility, so        that there is low risk of a hazard if the amine molybdate layer        is exposed to aqueous alkali and the amine is released

The amine molybdates are soluble in organic solvents, are compatiblewith film-forming solvent soluble organic binders, and give solutionsthat, when coated or printed on an inert substrate such as clear Mylarpolyester film and dried, form a continuous layer of the amine molybdatethat is substantially transparent to visible light. Such layers arethermally sensitive and find utility in thermographic materials and on3D objects for imaging by scanning laser or thermal printer. Clearlayers formed by means of the invention may also be useful on opaquesubstrates because they can impart desirable gloss, as distinct fromcompositions containing suspended insoluble molybdates that give mattsurfaces.

Imaging elements comprising these amine molybdates may be supported on aflexible sheet substrate, preferably a flexible transparent sheetsubstrate such as polyester. Alternatively a rigid 3D object substratemay be used such as the external surface of a container. There should bea good adhesive bond between imaging element and substrate. Thesubstrate should be able to withstand laser imaging of the element(comprising the amine molybdate) without unacceptable degradation ordeformation upon laser or thermal imaging. Preferred substrates aretransparent or translucent materials that absorb the IR radiation outputof the laser to some extent: otherwise the substrate may act as a heatsink to the laser-exposed areas of the imaging element, reducing layersensitivity. In this respect Mylar polyester film is better thanunfilled polypropylene or polyethylene.

The solvent-soluble molybdates used in the invention can be applied fromsolution and dried to give a near-transparent layer. Film-formingcompositions containing these amine molybdates give layers having goodadhesion transparency and imagewise thermal sensitivity. Such layers canhave filmogenic and transparency properties, e.g. on commercialtransparent polymer film supports such as clear polypropylene, providingnear-transparent, thermally sensitive sheet or web materials. Thesolvent-soluble amine molybdates also show good compatibility whenblended with specified organic solvent-soluble polymeric binders; theseblends can also form useful substantially transparent thermallysensitive layers, to provide thermographic materials.

The invention also provides amine molybdate compositions that, whenapplied as a solvent coating to commercially available transparent filmor supports or otherwise incorporated on or within transparent or semitransparent polymer layers, give direct thermally sensitive imagingmedia having excellent stability transparency and sensitivity propertiesfor thermal laser imaging or, if appropriate, thermal printing. Thecoating weight of the dry coating is normally in the range 0.5 to 20g/m², preferably 1 to 10 g/m².

The invention also provides thermally sensitive imaging materialscomprising a layer comprising the amine molybdate, adhering to asubstrate or within a substrate which is preferably an optically neartransparent or translucent polymeric material. Suitable substratesinclude paper, laminates and films of the type described above. Anotheraspect of this invention is thermally imageable materials comprising theamine molybdate and incorporated on a substrate.

Amine molybdates may also be useful in dispersed form in a thermographiclayer. Some are readily dispersed in water, and may be used, say, on anopaque substrate like paper to give a matt layer. Thus, depending on theconditions, the amine molybdates may be used for both transparent/glossymaterials and also opaque/matt materials.

Thermally imageable materials comprising an amine molybdate in solidsolution or dispersion in a molten material comprising a thermoplasticpolymer, may be made by cooling the material whilst rolling it flat orforming it into a shape, such as the shape of a container.

Thermally imageable materials comprising an amine molybdate in solutionor dispersion in a liquid photopolymerisable composition may be made byphotopolymerising the composition.

It will be appreciated by one of ordinary skill in the art that it ispossible to incorporate additives of various sorts in the imaginglayers, and which might be beneficial in certain circumstances. Suchadditives include, for example, polymer binders, mild reducing agents topromote thermal printer performance, colorants such as dyes or pigments,antioxidants and other known stabilisers, antiblocking materials such astalc or selected silicas, and materials adsorbent to or reactive withany thermolysis products of laser imaging.

An additive of particular utility, in solution or suspension or in aseparate layer, is an electron-donating dye precursor often known as acolour-former. When amine molybdates are incorporated in a layer withsuch colour-formers and thermally imaged, e.g. using a CO₂ laser,coloured images may be obtained. The colour may correspond to thatobtained by the use of common colour developers such as certain phenols.Weak block images may also be obtained, e.g. using a heat sealer at100-120 C and contact times of 1-10 seconds. Thus the amine molybdateacts as an electron acceptor and colour developer for at least some ofthese colour-formers. The low melting point of amine molybdates meansthat they can be fused with colour-formers, if desired.

Protective polymer or other layers on the imaging layer may be useful insome circumstances. For example, such layers may prevent or reducemechanical or chemical damage to the unexposed or exposed thermallysensitive layers of the invention. Layers comprising mild reducingagents may also be added to promote thermal printer performance. Suchlayers may also act to reduce emanation of any thermolysis products oflaser imaging. Such layers can be applied by known means such aslamination or coating.

As indicated above, an image can be formed by the application of heat.Preferably, heat is applied locally, on irradiation with a laser.Suitable lasers include those emitting at high energy, including Nd-YAGlasers and CO₂ lasers, the latter typically at a wavelength of 10,600nm. In many cases, it may be desirable to use a low-energy laser, suchas a diode laser, typically emitting light at a wavelength in the rangeof 800-1500 nm. In certain circumstances, this energy input may beinsufficient to cause the desired reaction, and the composition to beirradiated then preferably comprises a suitable absorbent material.

IR-absorbent materials are known. In general terms, any suitable suchmaterial may be incorporated, for the purposes of this invention, andcan be chosen by one of ordinary skill in the art. A particularlypreferred IR absorber for use in the invention is a conducting polymer,by which is meant a material that, in the polymerised state, compriseslinked monomers (typically rings) that are conjugated and which cantherefore allow delocalisation/conduction of positive or negativecharge. The conjugation allows an absorption shift that can becontrolled such that it applies to the wavelength of irradiation, andwhich may also depend on the concentration of the polymer.

Examples of monomers that can be conjugated to give suitable conductingpolymers are aniline, thiophene, pyrrole, furan and substitutedderivatives thereof. Such polymers, in addition to providing the desiredmeans of transferring heat from a low-power laser, have the advantagethat they do not readily diffuse out of the coating material. They canalso act as the polymer binder. Yet another advantage of such materialsis that they can be colourless, even at high loading (up to 5% byweight); this is by contrast to monomeric species that have been used,such as phthylocyanine, which absorb at about 800 nm but give thecomposition a greenish tinge, even at a loading of 0.1% by weight.

Depending on the components to be irradiated, a black or coloured imagemay be obtained. The colour may be dependent on the irradiation power;thus, for example, a blue colour may be overpowered to black.

Multi-colour printing may also be achieved, e.g. using differentcolour-formers (and, if necessary, absorbers) responsive to differentirradiation wavelengths. For example, UV, diode and CO₂ lasers may beused to give three-colour printing, by providing appropriate, differentcolour formers at different/overlapping locations on the substrate.

The initial colour of coating and image achieved on activation is notlimited. Theoretically, any initial or final colour (red, blue, green,etc) is achievable and the energy required to develop the image (e.g.100-140° C./2-4 Watts) can be controlled within a range. Additionally, astep-change of the image colour produced can be controlled withactivation (e.g. 150-200° C./3-5 Watts), and so more than one distinctcolour is possible from the same coating.

In general, the colour developer can be one or more of a range ofwater-compatible transition metal complex materials as an aminemolybdate.

The colour former can be one or more of a range of established basicdyes such as fluorans, phthalides etc.

The binder can be one or more of a range of water-soluble oramine-stabilised emulsion polymers, for a water-borne dispersion ink, ora solvent-soluble polymer for a solvent-borne dispersion or solutionink. Acrylic polymers can be used in each case.

Pigments can be water-dispersible inorganic or organic additives such ascalcium carbonate etc.

One or more of a range of additives can be utilised, includingsurfactants or lubricants such as zinc stearate etc.

The IR-sensitive coating can be applied by a range of methods such asflood coating, flexo/gravure etc.

The IR-sensitive coating can be applied to a range of substrates such asself-adhesive label etc.

A protective layer of a film-forming water-borne top-coat ink can beapplied onto the IR-sensitive coating.

The IR-absorber can be one or more of a range of water-compatibleorganic or inorganic materials, for a water-borne dispersion ink, or asolvent-compatible, organic or inorganic material for a solvent-bornedispersion or solution ink (in the latter case, the material ispreferably solvent-soluble).

The following Examples illustrate the invention.

EXAMPLE 1 Bis(2-ethylhexyl)amine octamolybdate

The following synthesis is adapted from the method given in U.S. Pat.No. 4,217,292 (Example 3) for dodecylammonium octamolybdate.

In a 500 ml flange flask vessel were weighed molybdenum trioxide (15.53g; Aldrich 99%; 10-20 μm particle size by Fisher sub-sieve sizer),deionised water (300 g) and ammonium chloride (8.6 g) (Aldrich reagent).The mixture was stirred vigorously while bis(2-ethylhexyl) amine (13.03g; Aldrich) was added dropwise over 10 minutes. The vessel contents werethen heated to reflux with stirring and refluxed for 4 hrs. A palegreen-blue tarry material formed that part adhered to the vessel walls.On cooling, the reaction mixture to room temperature, the tarry productformed a glass-like solid. The solid was collected by filtration withsome manipulative loss, ground and washed successively with deionisedwater and finally with isopropanol. Finally the pale green-blue productwas dried in an oven for 24 hrs at 65° C. Yield was 26.2 g. It wasreadily soluble in 2-butanone to give a pale-green solution. A trace ofwhite material (perhaps unreacted MoO₃) remained undissolved.

EXAMPLE 2 Coating Composition without Polymer Binder

Bis(2-ethylhexyl)amine octamolybdate (10 g) was dissolved in 2-butanone(30 g). The solution was separated from a trace of insoluble white solidimpurity to give a solution that can be used as a coating composition ofthe invention.

EXAMPLE 3 Thermally Imageable Material

The solution prepared in Example 2 was coated on each of four supports,i.e. opaque white (titanium dioxide-filled) Mylar film, clear Mylar(polyethylene terephthalate) film, domestic aluminium foil, andpolypropylene packaging film (UCB). This was done using a wire coatingbar, giving a 12 μm on wet film, and dried using warm air to give athermally imageable material.

Continuous glossy well-bonded films were obtained in each case. Thecoatings on clear Mylar and polypropylene were transparent and all werenon-tacky when cool. The dry coating weights were found to be about 3g/m². The resulting coated materials were exposed imagewise using a CO₂scribing laser beam of 0.3 mm diameter at a scan speed of 1000 mm/sec. Adistinct grey-black image of alphanumeric characters was obtained whenthe power was set at 3-4 Watts for Mylar and aluminium foil substrates.The images were less legible at 2 Watts, indicating sub-optimumexposure. With the polypropylene substrate, images were obtained atabout 6 W.

EXAMPLE 4 Coating Composition Containing Polymer Binder

A solution of bis(2-ethylhexyl)amine octamolybdate (10 g) was dissolvedin 2-butanone (30 g). The solution was separated from a trace ofinsoluble white solid impurity. 4 g of this solution was mixed with 4 gof a 15% by weight solution of Elvacite 2041 (a methyl methacrylatehomopolymer resin grade manufactured by INEOS) binder in 2-butanone togive a coating solution.

EXAMPLE 5 Thermally Imageable Film

The solution of Example 4 was coated on packaging grade polypropylenefilm using a wire-wound bar (giving a nominal 12 μm wet film thickness)and dried using warm air to give a transparent coated film. Thetransparency observed indicates good compatibility of the aminemolybdate and the acrylic binder. The dry coating weight was found to be2.8 g/m². The resulting coated film of the invention had hightransparency. It was exposed imagewise using a CO₂ scribing laser beamof 0.3 mm diameter at a scan speed of 1000 mm/sec. A distinct grey-blackimage of alphanumeric characters was obtained when the power was set at3-4 Watts. Some lifting of the image was observed at 4 Watts. The imagewas less legible at 2 Watts, indicating inadequate exposure.

EXAMPLE 6 Red Thermographic Film

To 0.4 g of a 25% by weight solution of bis(2-ethylhexylamine)octamolybdate in 2-butanone was added with thorough mixing 1.0 g of a33.3% by weight solution of Elvacite 2044 also in 2-butanone (Elvacite2044 is a n-butyl methacrylate-based acrylic resin manufactured by INEOSAcrylics). In this composition was dissolved by agitation 0.1 g of acommercial electron-donating colour-former (Pergascript Red I-6Bmanufactured by Ciba Specialty Chemicals and described as a bisindolylphthalide compound). The resulting pale yellowish-pink solution wascoated on clear Mylar film using a 25 wire bar and dried using warm air.A transparent film resulted.

A pale red image resulted on block imaging the film at 100° C. using aheat sealer and a contact time of 10 seconds. A distinct red imageresulted from imaging the film using a CO₂ scribing laser beam of 0.3 mmdiameter at a scan speed of 1000 mm/second and set at 3 Watts power.

EXAMPLE 7 Water-Borne Dispersion Inks

The effect of the presence of an IR absorber in an ink formulation ofthe invention was determined. Blue and red water-based acrylic-emulsioninks of PVOH-stabilised dispersion (comprising PBI2RN or PRI6B colourformer) were assessed.

A “standard” formulation of the invention was used, comprising thefollowing proportions of components (% w/w): Binder 16.0 Active Pigment7.0 Colour Former 7.0 Fluid 70.0

Various “active” formulations were used, each containing the IR absorberBaytron P (HC Starck), a conducting polymer. The proportions of IRabsorber used were 1.0, 2.5 and 5.0% (w/w). In, for example,formulations comprising 5.0% Baytron P, the composition was: Binder 15.2Active Pigment 6.7 Colour Former 6.7 Fluid 64.4 IR Absorber 5.0

The components were selected from: Binder Gohsenol GH-17 polyvinylalcohol and Texicryl acrylic emulsion; Active PigmentBis(2-ethylhexylamine)octamolybdate and di(cyclohexylamine)octamolybdateColour Former Pergascript blue I-2RN crystal violet lactone and redI-6B; Fluid water, dilute ammonium hydroxide etc; and IR AbsorberBaytron P

A 940 nm Rofin Dilas DF060 Diode Laser and K-bar 2.5-coated substrateswere used for image forming.

The results are shown in Table 1. A good image was obtained when BaytronP was present. TABLE 1 IR Level Imaged Ink Type Absorber (% w/w)Unimaged (940 nm) Standard, blue — n/a Off-white (slight) No image ″ —n/a ″ ″ Active, blue Baytron P 1.0 ″ ″ ″ ″ 1.0 ″ Blue Image ″ ″ 2.5 ″ ″″ ″ 2.5 ″ ″ ″ ″ 5.0 ″ ″ ″ ″ 5.0 ″ ″ Standard, red — n/a White No image ″— n/a ″ ″ Active, red Baytron P 1.0 Off-white (slight) Red Image ″ ″ 1.0″ ″ ″ ″ 2.5 ″ ″ ″ ″ 2.5 ″ ″ ″ ″ 5.0 ″ ″ ″ ″ 5.0 ″ ″

Samples of the blue ink formulations were coated with K-bar 2.5 ontoRafaltac Raflacoat (RC) and Hi-Fi polyester (PE) substrates. The coatedsubstrates were then used for Nd:YAG (1064 nm) laser text imaging. Twoformulations comprised Baytron P, two did not. The results are shown inTable 2. TABLE 2 IR Absorber at 5.0% Imaged Ink Type (w/w) SubstrateUnimaged (1064 nm) Standard, blue — RC White No Image Active, blueBaytron P RC Off-White (grey) Blue Text Standard, blue — PE White NoImage Active, blue Baytron P PE Off-White (grey) Blue Text

The coatings in which Baytron P was absent gave no image or very fainttext. PE-based samples gave better results than RC-based ones. Whereimages were obtained (i.e. when Baytron P was present), they were sharpand well-defined.

EXAMPLE 8 Solvent-Borne Dispersion Inks

Experiments similar to those of Example 7 were performed except thatsolvent-based inks were used.

The “standard” formulation was composed of (% w/w): Binder 21.7 ActivePigment 9.6 Colour Former 9.6 Fluid 59.1

The “active” formulations contained the IR absorber Iriodin LS820(Merck). The composition of the 5% (w/w) “active” formulation was:Binder 19.5 Active Pigment 8.6 Colour Former 8.6 Fluid 53.3 IR Absorber10.0

The results are shown in Table 3. Again, the presence of an IR absorberallowed image formation to occur. TABLE 3 Addi- Level tive % Imaged InkType Type w/w Unimaged (940 nm) Standard, blue — n/a Off-white (slightgreen) No image ″ — n/a ″ ″ ″ — n/a ″ ″ Active, blue Iriodin  5.0Off-white (grey/green) Blue Image LS820 ″ Iriodin  5.0 ″ ″ LS820 ″Iriodin  5.0 ″ ″ LS820 ″ Iriodin 10.0 ″ ″ LS820 ″ Iriodin 10.0 ″ ″ LS820″ Iriodin 10.0 ″ ″ LS820 Standard, red — n/a Off-white (pink) No image ″— n/a ″ ″ ″ — n/a ″ ″ Active, red Iriodin  5.0 Off-white (grey/pink) RedImage LS820 ″ Iriodin  5.0 ″ ″ LS820 ″ Iriodin 10.0 ″ ″ LS820 ″ Iriodin10.0 ″ ″ LS820 ″ Iriodin 10.0 ″ ″ LS820 ″ Iriodin 10.0 ″ ″ LS820

EXAMPLE 9 Solvent-Borne Solution Inks

Experiments similar to those of Examples 7 and 8 were performed exceptthat solvent-based inks in acrylic methyl ethyl ketone (MEK) solutionwere assessed.

The ink formulations comprised 0.1% (w/w) Pro-Jet 900NP (Avecia), an IRabsorber. Some formulations additionally comprised a UV absorber. Insome cases, colour former (CF) was present at a ratio of 1:1 or 1:2 withthe active pigment (CD). A typical formulation was composed of (% w/w):Binder 23.7 Active Pigment 4.6 Colour Former 4.6 UV Absorber 6.7 Fluid60.3 IR Absorber 0.1

The results are shown in Table 4. Generally, good images was obtained.TABLE 4 Ink Type Unimaged Imaged (940 nm) no CF Clear (green) Dark Image″ ″ ″ ″ ″ Incomplete Blue, CD:CF = 1:1 Clear (grey/green) Dark Image ″ ″″ ″ ″ Incomplete Blue, CD:CF = 1:2 ″ Dark Image ″ ″ ″ ″ ″ IncompleteRed, CD:CF = 1:1 Clear (grey/brown) Dark Image ″ Clear (green/grey) ″ ″″ Incomplete Red, CD:CF = 1:1 Clear (pink/brown) Dark Image ″ Clear(brown/grey) ″ ″ ″ Incomplete

1. A method for forming an image on a substrate, which comprises coatingthe substrate with a solution, in an organic solvent, of an aminecompound of molybdenum, tungsten or vanadium that changes colour onheating or irradiation, and heating or irradiating the coating.
 2. Amethod for forming an image on a substrate, which comprises coating thesubstrate with an aqueous dispersion or suspension, of an amine compoundof molybdenum, tungsten or vanadium that changes colour on heating orirradiation, and heating or irradiating the coating.
 3. The methodaccording to claim 1, wherein the amine compound is of molybdenum (VI).4. The method according to claim 3, wherein the amine is a secondary ortertiary alkylamine in which each alkyl group has up to 12 carbon atomsand the amine has up to 24 carbon atoms.
 5. The method according toclaim 1, wherein the coating also comprises the use of an organicpolymer binder.
 6. The method according to claim 1, wherein the coatingalso comprises the use of a colour-former.
 7. The method according toclaim 1, wherein the substrate is substantially transparent to visiblelight.
 8. The method according to claim 1, wherein the coating isirradiated using a laser.
 9. The method according to claim 8, whereinthe laser light has a wavelength of 800-1500 mm.
 10. The methodaccording to claim 8, wherein the coating additionally comprises an IRabsorber that absorbs laser radiation.
 11. A coated substrate, whereinthe coating is a substantially visible light-transparent layercomprising an amine compound of molybdenum, tungsten or vanadium thatchanges colour on heating or irradiation.
 12. The coated substrateaccording to claim 11, wherein the coating also comprises an organicpolymer binder.
 13. The coated substrate according to claim 11, whereinthe substrate is also substantially transparent to visible light. 14.The coated substrate according to claim 11, including also an imageformed therein by heating or irradiation.
 15. The coated substrateaccording to claim 11, wherein the coating additionally comprises an IRabsorber that absorbs laser radiation.
 16. A solution of an aminecompound of molybdenum, tungsten or vanadium that changes colour onheating or irradiation and one of the following: a thermoplasticpolymer; or a photopolymerisable monomer.
 17. (canceled)
 18. Thesolution according to claim 17, which is fluid at or below 150° C. 19.The method according to claim 2, wherein the amine compound is ofmolybdenum (VI).
 20. The method according to claim 19, wherein the amineis a secondary or tertiary alkylamine in which each alkyl group has upto 12 carbon atoms and the amine has up to 24 carbon atoms.
 21. Themethod according to claim 2, wherein the coating also comprises the useof an organic polymer binder.
 22. The method according to claim 2,wherein the coating also comprises the use of a colour-former, e.g. asubstantially colourless electron-donating dye precursor.
 23. The methodaccording to claim 2, wherein the substrate is substantially transparentto visible light.
 24. The method according to claim 2, wherein thecoating is irradiated using a laser.
 25. The method according to claim24, wherein the laser light has a wavelength of 800-1500 nm.
 26. Themethod according to claim 24, wherein the coating additionally comprisesan IR absorber that absorbs laser radiation.
 27. The coated substrateaccording to claim 11, wherein the coating also comprises the use of acolour-former.
 28. The solution according to claim 16, wherein the aminecompound is of molybdenum (VI).
 29. The solution according to claim 16,wherein the amine is a secondary or tertiary alkylamine in which eachalkyl group has up to 12 carbon atoms and the amine has up to 24 carbonatoms.